1. Technical Field
The present invention relates to novel topically applicable cosmetic and/or dermatological compositions comprising depigmenting agents for treating the skin of the face and/or body for the purposes of lightening the skin, evening skin tone and/or treating areas of hyperpigmentation. More specifically, the depigmenting agents are inhibitors of Type I H+, K+-ATPases.
2. Description of the Prior Art
Consumers of skin lightening products spend more than $1 billion annually in search of skin with an even tone on their faces, hands and bodies. The development of areas of hyperpigmentation on the skin is obviously of great concern to these individuals. The hyperpigmented areas are caused by a concentration of melanin in the keratinocytes located at or near the skin surface. Melanin pigment is produced in melanocytes in highly specialized organelles known as melanosomes. Melanocytes are found in several locations throughout the body, including in the bottom layer of the skin's epidermis, the iris of the eye and the hair. Manufacturing of melanin begins when melanin-making enzymes are activated and transform the amino acid tyrosine to intermediates of the end product, melanin. The actual production of melanin begins in the melanosomes. Inside human melanosomes, a series of chemical reactions, catalyzed by enzymes, converts tyrosine into two types of melanin, eumelanin, which is brown or black in color, and pheomelanin, which is red or yellow. The mechanism of formation of melanin includes the following principal mechanisms:

Tyrosinase is the essential enzyme involved in this reaction sequence. It catalyzes the conversion of tyrosine into dopa (dihydroxyphenylalanine) and the conversion of dopa into dopaquinone.
Once the melanosomes are loaded with melanin, the melanosomes are transported along a secretory pathway to their final destination in keratinocytes, which are barrier cells in the uppermost layer of the skin, and into the hair, and to other locations in the body. The amount of melanin transported and the mix of the pigments determines skin, eye and hair color in humans. Melanin functions to protect DNA in skin cells by absorbing ultraviolet radiation which can damage the DNA, and leave the skin vulnerable to cell damage, including sunburn, premature aging and skin cancer.
Various depigmenting agents having differing mechanisms of action and levels of efficacy are known. Depigmenting agents may act directly on epidermal melanocytes, such as by destroying these cells. One such agent is hydroquinone and its derivatives. Hydroquinone also competes for tyrosine oxidation in active melanocytes. Although highly efficacious as depigmenting agents, the use of these compounds, in view of their cytotoxicity, is legally limited to a concentration of 2% without a prescription in the U.S., and is not available over the counter elsewhere. Examples of other depigmenting agents include kojic acid, which chelates the copper ion in the active site of tyrosinase; but which tends to be unstable in the processing of cosmetics; hydrogen peroxide, which inhibits melanogenesis because it bleaches the melanin but which is unstable; ascorbic acid, which converts dopaquinone back to dopa, but which has low activity and low stability; salicylic acid and lactic acid, which increase cell turnover; and unsaturated fatty acids, such as linoleic acid, which affect the processing and function of tyrosinase in connection with the ubiquitin-proteasome pathway.
Other depigmenting agents include those which interfere with one or more steps in the production of melanin. These agents may act by inhibiting one or more enzymes (e.g. tyrosinase) involved in melanogenesis or by inserting themselves in the synthetic chain as a structural analogue of one of the chemical compounds. Still other depigmenting agents may act by disrupting tyrosinase processing and sorting through the secretory pathway (translocation through membrane-bound organelles, e.g., endoplasmic reticulum→Golgi→endosomes→melanosomes in melanocytes). A further depigmenting mechanism could involve the modulation of tyrosinase messenger RNA (mRNA) transcription and its post-transcriptional stability. Depigmenting agents may also act by decomposing already formed melanin.
During routine screening of compounds for inhibition of melanogenesis in cultured B16F10 mouse melanoma cells, it was unexpectedly discovered by the inventors that a class of compounds called substituted benzimidazoles all strongly inhibited melanogenesis. This was quite surprising since the only activity known for these compounds is the specific inhibition of the proton pump protein reportedly only found in the apical cytoplasmic membrane of gastric parietal cells (Olbe, L., Carlsson E., Lindberg P. A Proton-Pump Inhibitor Expedition: The Case Histories of Omeprazole and Esomeprazole, Nature Reviews Drug Discovery, 2:132-9, 2003). The gastric proton pump has never been found in melanocytes and the inventors were unable to detect its gene expression in melanocytes. Another gastric proton pump inhibitor, a substituted imidazopyridine compound with a different reactive site, was tested and it also surprisingly inhibited melanogenesis. This led the inventors to consider, for the first time, using gastric proton pump inhibitors to depigment skin.
Recent studies have suggested that differences in epidermal pigmentation may be due to differences in melanosomal pH. However, the literature has been contradictory as to whether melanogenesis is favored by acidic or basic pH. On the one hand, it has been observed that melanosomes are normally acidic (Brilliant, M. and Gardner, J.: Melanosomal pH, Pink Locus Protein and their Roles in Melanogenesis, J. of Invest. Dermatol. 117(2) 2001; Moellmann, G., Slominski, A., Kuklinska, E., Lerner A. B.: Regulation of Melanogensis in Melanocytes. Pigment Cell Res., 1:79-87, 1988; Bhatnagar, V., Anjaiah, S. Puri, N, Arudhra Darshanam, B. N., and Ramaia, A.: pH of Melanosomes of B16 Murine Melanoma is Acidic: Its Physiological Importance in the Regulation of Melanin Biosynthesis, Arch. Biochem. Biophys. 307:183-192, 1993; Ramaiah, A.: Lag Kinetics of Tyrosinase: Its Physiologic Implications, Indian J. Biochem. and Biophys. 33:349-356, 1996), and that the acidification of various intracellular compartments is important for a number of processes (Van Dyke, R. W.: Acidification of Lysosomes and Endosomes, Sub-Cellular Biochem., 27:331-360, 1996; Grabe, M. and Oster, G.: Regulation of Organelle Acidity, J. General Physiol. 117:329-344, 2001). Devi et al. proposed that since melanosomes can be acidic, low melanosomal pH facilitates melanogensis, and therefore tyrosinase activity is optimal at acidic pH and inactive at neutral pH (Devi, C. C., Tripathi R. K., Ramaia, A, pH-dependent Interconvertible Allosteric Forms of Murine Melanoma Tyrosinase: Physiological Implications. Eur. J. Biochem. 166:705-711, 1987). Very recently, Gunathilake, et al. reported that melanocytes, and particularly the dendrites, from darkly pigmented subjects are significantly more acidic than those from lightly pigmented subjects, and that this acidity appears to be localized to melanosomes (Gunathiliake R., Schurer N., Shoo B., Celli, A., Hachem J. P., Curmrine D., Sirimanna, G., Feingold K., Mauro t., Elias P.: pH-regulated Mechanism Accounts for Pigment-Type Differences in Epidermal Barrier Function. J. Invest. Dermatol, 129:1719-1729, 2009). On the other hand, other groups have observed that mammalian tyrosinase has optimal enzymatic activity at near neutral pH and that its activity is lost with decreasing pH (Hearing, V. J. and Ekel, T. M.: Mammalian Tyrosinase. A Comparison of Tyrosine Hydroxylation and Melanin Formation, J. Biochem., 157:549-557, 1976; Saeki, H. and Oikawa, A.: Stimulation of Ionophores of Tyrosinase Activity of Mouse Melanoma Cells in Culture, J. Investig. Dermatol. 85:423-425, 1985; Townsend, D., Guillery, P., and King. R. A.: Optimized Assay for Mammalian Tyrosinase (Polyphenol Phenyloxidase), Anal. Biochem. 139:345-352, 1984). Ancans et al., reported that near neutral melansomal pH is optimal for human tyrosinase activity, melanogenesis and maturation rate of melanosomes, and that low pH suppresses melanin production in Caucasian melanocytes. It was further observed that the ratio of eumelanin/phaeomelanin production and the maturation rate of melanosomes are regulated by melanosomal pH, and that therefore, melanosomal pH appears to be an essential factor which regulates multiple stages of melanin production (Ancans, J, D., Tobin, J., Hoogdujin, J. J. Smit, N. P., Wakamatsu, K., and Thody, A. J.: Melanosomal pH Controls Rate of Melanogenesis, Eumelanin/Phaeomelanin Ratio and Melanosome Maturation in Melanocytes and Melanoma Cells, Experimental Cell Research 268:26-35, 2001). Studies by Smith et al. also suggested that the internal pH of melanosomes in Caucasians is acidic, and at this pH tyrosinase is inactive, while the pH of melanosomes of Blacks appears to be more neutral and optimal for tyrosinase activity (Smith et al.: The Relationship Between Na+/H+ Exchanger Expression and Tyrosinase Activity in Human Melanocytes. Exptl. Cell Res. 298:521-534, 2004). Thus, there is disagreement in the literature as to the role of melanosome pH in the production of melanin.
Puri et al. reported the aberrant pH of mouse “p” gene (pink-eyed dilution (p) mutant) melanocytes, and, based on a finding of fewer acidic melanosomes, hypothesized that the p protein functions in the acidification of melanosomes, e.g., an ion-exchange or channel protein, in the melanosomal membrane, which may affect the activity and/or routing of tyrosinase (Puri, N., Gardner, J. M., Brilliant, M. H.: Aberrant pH of Melanosomes in Pink-eyed Dilution (p) Mutant Melanocytes. Soc. Invest. Dermatol. 115:607-613, 2000). Ancans et al. suggested alternative hypotheses to Puri, since p-protein does not utilize energy from ATP which would enable it to function as an ionic transporter against a proton gradient. Ancans et al. treated mutant and wild-type melanosomes with v-type proton pump inhibitors (responsible for organelle acidification), and observed that, in mutant cells, neutralization resulted in increased melanin content, while there was no significant change in the wild-type cells. The study suggested that P-locus protein has a role in creating a near neutral local microenvironment and that this change facilitates tyrosinase activity. Thus, p-locus protein may function as a channel to reduce the proton concentration inside the melanosome analogous to Na+/H+ antiporters (NHEs), (Ancans, J., Hooduijn, J., Thody, A. J.: Melanosomal pH, Pink Locus Protein and their Roles in Melanogenesis. J. Invest. Dermatol. 117(1):158-159, 2001). Halaban et al., suggested that bafilomycin A1 and monensin play dual roles in the processing of tyrosinase: reduction of levels of tyrosinase retained in the endoplasmic reticulum and facilitating the release of tyrosinase from the endoplasmic reticulum to the Golgi by increasing the pH in either the endoplasmic reticulum or the endoplasmic reticulum-Golgi intermediate compartment (Halaban, R., Patton, R. S., Cheng, E., Svedine, S., Trombetta, E. S., Wahl, M. L., Arujan, S. and Hebert, D. N.: Abnormal Acidification of Melanoma Cells Induces Tyrosinase Retention in the Early Secretory Pathways, J. Biol. Chem. 277(17):14821-14828, 2002).
Thus there is no clear guidance from the literature as to whether increasing or decreasing the pH of acidic organelles including melanosomes would benefit depigmentation. Even if one hypothesized that agents which inhibit the neutralization of pH in melanocytes might be desirable, prior to the present invention, there was no recognized means by which to reduce the pH of the acidic organelles, and therefore, certainly none that were safe. The available pH adjusting compounds such as bafilomycin A1 and monensin increase the pH of acidic organelles, and the known target for the gastric proton pump inhibitors is not present in melanocytes.